Stains derived from various staining substances and finger prints will attach on optical disks such as a playback-only optical disk, optical recording disk and a magneto optical recording disk, when they are used. Such attachment of stains or finger prints is undesirable and optical disks may be given an appropriate surface treatment to the surface for improving antifouling property, reducing finger-print attachment property or increasing finger-print removing property. For example, giving various water/oil repellent treatments to the surface of optical disks is being examined.
For the purpose of improving the scratch resistance of the surface of an optical information medium, it is general practice to form a transparent and scratch-resistant hardcoat on the surface of the record and/or playback beam entering side in the medium. The hardcoat is formed by coating on the surface of a medium an active energy ray polymerizing curable compound having 2 or greater polymerizing functional groups such as (meth)acryloyl in the molecule, which is allowed to cure by irradiation of an active energy ray such as an ultraviolet ray. However, such a hardcoat is given only for improving the scratch resistance and therefore not expected to have an antifouling effect on dust, atmospheric oil mist or staining substances such as finger print stains.
The hardcoat having an antifouling property against organic stains is proposed Japanese Publication JP-A-10-110118 which proposed that non cross-linking fluorochemical surfactants are kneaded into hardcoat agents. Non cross-linking fluorochemical surfactants do not have a polymerizing double bond or will not cross-link base resin used in hardcoat agents.
Japanese Publication JP-A-11-293159 also proposes that both non cross-linking fluorochemical surfactants and cross-linking fluorochemical surfactants are kneaded into hardcoat agents. Cross-linking fluorochemical surfactants include fluorinated alkyl(meth)acrylates such as perfluorooctylethyl(meth)acrylate, hexafluoropropyl(meth)acrylate and octafluoropentyl(meth)acrylate. These cross-linking fluorochemical surfactants are of a polymerizing double bond and fixed by cross-linkage with a base resin of hardcoat agents.
Japanese Publication JP-A-11-213444 has disclosed that fluorinated polymers are coated on the surface of optical disk base plates such as conventional polycarbonates
Japanese Publication JP-A-11-503768 has disclosed a radiation curable composition consisting of a fluorinated urethane oligomer and diluent monomer.
Japanese Publication JP-A-2002-190136 has disclosed that metal chalcogenide microparticles such as silica microparticles are allowed to be contained in a hardcoat to increase the scratch resistance of the hardcoat and also the film of a silane coupling agent containing a water-repellent group or oil-repellent group is provided on the hardcoat to impart an increased antifouling property to the surface of an optical information medium.
When the friction coefficient of the optical information medium is reduced, since it is possible to slide and avoid impact that occurs with contact with a hard projected substance, the development of scratches can be controlled. Therefore, it is desirable to reduce the friction coefficient on the hardcoat surface to improve the scratch resistance. In particular, Blu-ray Disc is now commercially available in which the numerical apertures (NA) of objective lens for focusing record/playback laser light is increased to approximately 0.85 and the entering wave of record/playback laser light is decreased to approximately 400 nm to make a diameter of light focusing spot of laser light smaller, thereby attaining a recording capacity more than 4 times greater than that of DVD. As described above, an increased NA makes smaller the working distance between the objective lens and the surface of an optical information medium, (for example, in the case of NA=approximately 0.85, the working distance is approximately 100 μm, which is extremely narrow as compared with the conventional case), thus making it extremely likely that the surface of the optical information medium may contact with the objective lens or a supporting substrate of the lens, while the optical information medium is rotating. Therefore, it is necessary to increase the wear resistance on the hardcoat surface and also reduce the friction coefficient.
Further, regarding factors other than the optical disk, the scratch resistance and wear resistance must be imparted to various display elements such as an optical lens, optical filter, anti-reflection film, liquid crystal display, CRT display, plasma display and EL display. Inmost cases, a protective layer (hardcoat layer) is imparted to their surfaces. As with an optical disk, stains derived from various staining substances and finger prints will attach to the surface of these substances, while they are used. Attachment of these stains and finger prints is not desired, which necessitates development of a hardcoat layer excellent in antifouling property.
However, the above-described conventional technology has physical problems that the hardcoat is insufficient in antifouling property and lower in hardness, in addition to a higher production cost.